Deepest Underground

Deepest Underground is a subarea below Sand Kingdom. There are 2 ways to enter this subarea, by going into the hole under the floating piramid or by falling into a hole somewhere in the desert. It is the place where the treasure-ring was kept, until Bowser stole it. The subarea has 2 parts: one where you need to parcour with a slippery icey floor and one where you fight the boss called Knucklotec. When you beated the game and there is world peace, the treasure-ring will re-appear in the Deepest Underground, on the plateau it was kept on before it got stolen.